The present invention relates to food packaging containers for use in packaging, for example, desserts, foods to be cooked or cooked foods.
Conventional food packaging containers were prepared from a laminate sheet which was composed of aluminum foil and a synthetic resin layer joined to one surface of the foil. The aluminum foil had a greater thickness than the synthetic resin layer.
The shape retentivity of such conventional containers is dependent on the material characteristics of the aluminum foil. The containers therefore had the the problem of being inferior in impact resistance and compression resistance and susceptible to deformation during transport or handling.
To overcome this problem, we carried out extensive research and developed laminate sheets comprising aluminum foil and a synthetic resin layer which conversely had a greater thickness than the foil, along with various attempts to prevent to the greatest possible extent the deformation of containers prepared from these sheets. However, our research revealed another problem that when the blank was subjected to deep drawing, the flange of the resulting container wrinkled locally.
If an increased blank holder pressure was applied to the flange to eliminate the wrinkling, use of common synthetic resin film encountered the following problem since the properties of the film differ in different directions. Although no wrinkle then occurred in the flange, the material was drawn unevenly circumferentially of the flange, so that the drawing operation produced variations in the width of the flange, causing a fracture in the direction in which the material was smaller in elongation. To be sure, the flange can be made free of wrinkling and planar by increasing the blank holder pressure, whereas the flange is not uniform in the residual strain of the material. This resulted in the problem that for example when the container obtained was heated for sterilization with retorted food placed therein, the flange wrinkled in an undulating or wavy fashion and became deformed.